Notes from RR Construction

Published in 1922, Railroad Construction Theory and Practice – A Text-Book for the use of students in colleges and technical schools and a hand-book for the use of engineers in field and office by Walter Loring Webb, C.E.

While researching how to build a model railroad wooden trestle I decided to gather some of the bits and pieces pulled from the book that I thought might be useful – something I could refer back to easier then looking through the book.

Pg 207 §171. Multiple-story construction
Since the bents of any trestle are usually of variable height and those heights are not always an even multiple of the uniform height desired for the stories, it becomes necessary to make the upper stories of uniform height and let the odd amount go to the lowest story.
Pg 208 §172. Span
The spans generally used vary from 10 to 16 feet. The Norfolk and Western R.R. uses a span of 12’6″ for all single story trestles, and a span of 25′ for all multiple-story trestles. The stringers are the same in both cases but when the span is 25 feet, knee-braces are run from the sill of the first story below to near the middle of each set of stringers. These knee-braces are connected at the top by a “straining-beam” on which the stringers rest, thus supporting the stringer in the center and virtually reducing the span about one-half.
Pg 209 §173. Foundations
RC_Fig-80Stone foundations are the best and the most expensive. For very high trestles the Norfolk and Western R.R. employs foundations as shown in Fig. 80, the walls being 4 feet thick. When the height of the trestle is 72 feet or less the foundation is made continuous.

Note: Note the use of Mud-sills under the sill proper. The text mentions that the Louisville and Nashviller R.R. used eight blocks 12″ x 12″ x 6′ under each bent. The ones in the drawing appear to me to be more like 12″ x 6″. Still .. another detail.

Pg 210 §173. Longitudinal bracing
This bracing generally consists of horizontal “waling-strips” and diagonal braces. Sometimes the braces are placed wholly on the outside posts unless the trestle is very high. For single story trestles the P.R.R. employs the “laced” system, i.e., a line of posts joining the cap of one bent with the sill of the next (…) Connecting these braces in the center more than doubles their columnar strength. Diagonal braces, when bolted to posts, should be fastened to them as near the ends of he posts as possible. The sizes employed vary largely, depending on the clear length and on whether they are expected to act by tension or compression. 3″ x 12″ planks are often used when the design would require tensile strength only, and 8″ x 8″ posts are often used when compression may be expected.
Pg 216. Drawing showing Knee-Braces
This drawing .. Plate II shows the Norfork & Western standard trestle design with knee-bracing as of Sept 10, 1891
Pg 220. §Loading
As shown in §172, the span of trestles is always small, is generally 14 feet, and never greater then 18 feet except when supported by knee-braces. The greatest load that will ever come on any one span will be the concentrated loading of the drivers of a very heavy locomotive. With spans of 14 feet or less it is impossible for even the four pairs of drivers to be on the same span at once.

[Note: This is of course referring to standard gauge locomotives. The author seems to assume a generic 5-foot wheelbase. This is not always the case with narrow gauge locomotives – but then the loading is much less for them also.]

(…) Disregarding all refinements as to actual dimensions, the ordinary maximum loading for standard-gauge railroads may be taken as that due to four driving-axles, spaced 5’0″ apart and giving a pressure of 40,000 pounds per axle. this should be increased to 54,000 pounds per axle (same spacing) for the heaviest traffic. On the basis of 40,000 pounds per axle or 20,000 pounds per wheel the following results have been computed: This loading is assumed to allow for impact.

[Note: USRA standard designs for 0-8-0 switchers, 4-8-2 Mikado and 4-8-2 Mountain in light versions with an axle load of 54,000 pounds and a heavy version with an axle load of 60,000 pounds. It seems that what was considered light and heavy changed as time passed. The USRA ‘Light Mikado’ is listed at 220,000 pounds. The wheelbase on the USRA 0-8-8 locomotive was 15 ft – which helps explain the earlier statement that a trestle span of 14 ft you can’t get all four drivers on the span – and also with a 15 ft wheelbase there is indeed a 5′ spacing between axles.]

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